The present invention relates to a method of and device for magnetic-abrasive machining of parts.
The magnetic-abrasive machining of parts does not require that the accuracy of a machine tool be less than the specific accuracy of a workpiece. The realization of any precise geometrical form of workpiece is caused by a unique material removal mechanism which is disclosed for example in "Mechanism of Material Removal in the Magnetic Abrasive Process and the Accuracy of the Machining", G. Z. Kremen, et al., Int. J. Prod. Res., 1996, Volume 34, No. 9, Pages 2629-2638. This unique material removal mechanism, in addition to the accuracy of machining, to allow also machining hard and brittle materials, (ceramic, silicone) without their destruction. The other unique property of the process is that the peaks of the grains enter into the valleys of matching uneveness of the workpiece (see FIG. 2a of the same publication), and the material removal rate is higher close to the top of the peak than that at the bottom of the valley. As a result the surface of the parts machined with this method exhibits large ratios of length to amplitude. In other words, unevenesses of the surface do not have sharp edges. It is known that such a characteristic of the surface provide for a greater strength of the part, which is especially important for hard and brittle materials, such as ceramics, silicon, glass, etc.
Some methods and devices for magnetic-abrasive machining of parts are disclosed in U.S. Pat. Nos. 5,569,061 and 5,775,976. In this methods and devices, however, the material removal rate is low when compared with regular grinding. Also, non-magnetic parts can be machined when they have a small thickness, no more than 10-15 mm, while magnetic parts can be machined when they have a thickness of 100-120 mm. These disadvantages are connected with magnetic-properties of the parts to be machined and the magnetic-abrasive powders. Magnetic parts and powders contain steel which can not be magnetized more than its saturation limit about 2 T. The force of attracting a powder grain in the working gap is in cubic relation to the magnetic field strength, or in other words the magnetization of steel determines the material removal rate.
The magnetic-abrasive machining, in addition to the above mentioned two disadvantages has two properties which determine the efficiency of the process. First of all the powder must be pressed to the part to be machined, since if it is not pressed to the part there is no cutting force and therefore a cutting process. Secondly, the magnetic field must retain the powder in a working gap when the part is being machined and when it is not being machined.
In conventional magnetic-abrasive machining there is a contradiction in that, in order to increase the material removal rate it is necessary to increase a force which presses the powder to the part or in other words a cutting force. However, the increase of the cutting force leads to an increase in a friction force of the powder against the part. The magnetic force which retains the powder in the gaps has however a certain limit. The increase of the pressing force or cutting forces can lead to the situation that the friction force of the powder against the parts force the powder out of the working gap. In order to increase the material removal rate, it is proposed in U.S. Pat. Nos. 5,569,061 and 5,775,976 to change the nature of the force which presses the powder against the part. Hydrodynamic and aerodynamic forces are used in the solutions disclosed in these references. However, in order to use these forces it is necessary to provide additional devices which generate a jet and vacuum and supply the same into the cutting zone. In addition, the devices must be located in the machining zone, which makes difficult the operator's work. Also, the use of these devices requires high skills.
The magnetic-abrasive machining disclosed in this references includes the utilization of poles located opposite to one another. This approach has a certain limitation with regard to the diameter of the parts to be machined both magnetic and non magnetic. The magnitude of a gradient and the magnitude of the magnetic field in the working gap are related to one another. It has been determined experimentally that in order to retain the magnetic-abrasive powder in a working gap it is necessary to provide the magnitude of the magnetic field not less than B=0.8 T. When the magnetic is less than 0.8 T and the speed of rotation of the part is more or equal to 1 ms, the magnetic-abrasive powder is not retained and is forced out. The reason is that with the increase of the distance between the poles even for machining of a magnetic part, the field in the working gaps is reduced, and with the diameter of part more than 100-120 mm drops to B=0.8 T and less. This takes place under the condition that the external magnetic field is equal to 0.8-1.2 T or the powder and the parts are both magnetized into a saturation induction. In other words, the parts with the diameter more than 100-120 mm can not be machined with this method.
In the method disclosed in U.S. Pat. No. 5,813,901, FIG. 1, during the machining of a non-magnetic part, the magnetic-abrasive powder is not attracted to the part, but instead is attracted to the pole tips. With the maximum magnitude of the ferromagnetic field about 2 T and distance (gap) between the pole tips about 10-15 mm, the field drops below B=0.8 T. With this magnitude of the field, the magnetic-abrasive powder is not retained in the gap, if a non-magnetic part is introduced and attempted to be machined. In other words, the utilization of the oppositely located pole tips is limited by the possibility of machining of magnetic parts only in a diameter of not more than 100-120 mm, and non-magnetic parts with a diameter of 10-15 mm. The material removal rate is limited by a value of magnetization of steel in the part to be machined and the magnetic-abrasive powder.
Another method of magnetic-abrasive machining of both surfaces of even non-magnetic workpiece is disclosed in Japanese patent no. 62-39172. It also utilizes the oppositely located pole tips. However, here the parts to be machined are rotated, and the pole tips are rotated. This method has the disadvantages which are the same as in the above analyzed patents. It is not possible to machine non-magnetic parts with the thickness of more than 10-15 mm, and magnetic parts with the thickness of more than 100-120 mm. The material removal rate is limited by a magnitude of magnetization of the steel part and of the steel in the magnetic-abrasive powder.
U.S. Pat. No. 4,821,466 discloses a method in which the non-magnetic abrasive grains in a magnetic fluid are pushed to a workpiece with a floating pad being given a buoyant force. This patent has the same limitation with respect to the pressing of the powder to the part by a magnetic field. The material removal rate is limited by the magnitude of magnetization of steel powder in the magnetic fluid.